The present invention relates to a new and improved construction of a distance protection apparatus for electrical lines or networks.
Generally speaking, the distance protection apparatus for electrical lines, of the present development, is of the type comprising a starter circuit responsive to at least one predetermined fault criterion, and at least one measuring circuit whose characteristic, in the complex impedance plane, can be switched between a number of distance-trigger or trip boundaries. This switching of the characteristic of the measuring circuit and the distance measuring system contained therein corresponds to the generally known staggered protection operation. During performance thereof, the distance-trip boundaries and thus the boundary limits at the line to be protected are increased in steps or increments after successive time-intervals for the response or non-response of the measuring circuit, until the response of the measuring circuit has been accomplished by producing a trigger or trip signal. The trip signal, which causes shutdown of the line section at which there appears the fault, is thus dependent upon the presence of an excitation signal delivered by the starter circuit, i.e. a signal indicating that the fault has not yet been turned-off by another protection station and thus has been overcome. In this way there can be achieved the result that, from a number of protection stations, whose maximum trigger or trip region covers a predetermined line or network region, that protection station always initially responds where the fault lies within the zone boundaries closest to the relevant station. In this way there is realized minimumization of the line or network region affected by the switching-off operation and there is achieved as rapid as possible shutdown of the faulty line section.
In the final stage of a measuring circuit or a measuring system the trip boundaries which, in the complex impedance plane intersect the reactance axis and the resistance axis (reactance boundary and resistance boundary, respectively), are generally shifted to infinity, so that--to the extent present--only the trigger boundaries of a directional element (directional boundaries) passing through the coordinate null point or passing such at only a slight spacing become effective. The reactance, resistance and directional-boundaries moreover can be united into a uniform, geometrically defined line course, for instance, with the known measuring circuits having circular characteristic, in other words in contrast to the likewise known polygonal or quadrilateral characteristics where the aforementioned boundaries are each formed by a line section.
In the case of measuring circuit-end stages without any directional element, so-called non-directed or non-directional end stages, which cause purely a release or switching-through of the excitation signal as a trip signal, and also with the heretofore known end or final stages having conventional directional elements, extreme, but in fact still permissible loads, only then do not cause a faulty or spurious tripping operation when the excitation circuit, i.e. the fault-protection element of the starting circuit, can decide with adequate accuracy and reliability between such extreme load conditions and short-circuits which constitute real faults. The probability of spurious tripping is thus only conditionally limited with the heretofore known circuits and only can be reduced by using comparatively great circuit expenditure.